feat(lib): port libghostty, Ghostty's embedded C API

Ports apprt/embedded.zig, main_c.zig and config/CApi.zig + c_get.zig as
crates/ghostty-lib (cdylib/staticlib/rlib `libghostty`), exporting every
function of the unchanged include/ghostty.h - the ABI Ghostty's Swift
macOS/iOS app links against. The embedded runtime implements the
runtime traits over the host's C callbacks, so the real core drives it.

- ghostty-core selects the renderer per target: Metal on Apple (surface
  NSView/UIView via RtSurface::platform_view), OpenGL elsewhere.
- ghostty_config_get follows Zig's c_get rules via a CValue type (143
  supported keys, checked against the Zig field types).
- The inspector C API is complete; its Metal backend replaces ImGui's
  Metal renderer and has not run.

Verification: generated C ABI probe (sizeof/alignof/offsetof of every
struct/union, 755 enum values, every function linked, nm exports, headers
byte-identical to upstream); a C host on Linux covering init, CLI
actions, config, app, keys, focus and color scheme; an opt-in
test-platform feature creating real surfaces on Linux with headless EGL
and a pty (13 tests). Apple targets are clippy-clean but not run; the
Swift app has not been built against it.

Test Plan:
- cargo test -p ghostty-lib (and --features test-platform)
- cargo clippy --target aarch64-apple-darwin -p ghostty-lib (cross env
  in docs/rewrite/notes/macos-M1.md)

crates/ghostty-config/src/testdata/c_get_supported.txt

@@ -0,0 +1,143 @@
+language
+font-synthetic-style
+font-size
+font-thicken
+font-thicken-strength
+font-shaping-break
+alpha-blending
+grapheme-width-method
+freetype-load-flags
+background
+foreground
+background-image
+background-image-opacity
+background-image-position
+background-image-fit
+background-image-repeat
+selection-clear-on-typing
+selection-clear-on-copy
+minimum-contrast
+palette
+palette-generate
+palette-harmonious
+cursor-opacity
+cursor-style
+cursor-style-blink
+cursor-click-to-move
+mouse-hide-while-typing
+scroll-to-bottom
+mouse-shift-capture
+mouse-reporting
+background-opacity
+background-opacity-cells
+background-blur
+unfocused-split-opacity
+unfocused-split-fill
+split-divider-color
+split-preserve-zoom
+notify-on-command-finish
+notify-on-command-finish-action
+notify-on-command-finish-after
+wait-after-command
+abnormal-command-exit-runtime
+scrollbar
+link-url
+link-previews
+maximize
+fullscreen
+title
+class
+x11-instance-name
+window-padding-balance
+window-padding-color
+window-vsync
+window-inherit-working-directory
+tab-inherit-working-directory
+split-inherit-working-directory
+window-inherit-font-size
+window-decoration
+window-title-font-family
+window-subtitle
+window-theme
+window-colorspace
+window-height
+window-width
+window-position-x
+window-position-y
+window-save-state
+window-step-resize
+window-new-tab-position
+window-show-tab-bar
+window-titlebar-background
+window-titlebar-foreground
+resize-overlay
+resize-overlay-position
+resize-overlay-duration
+focus-follows-mouse
+clipboard-read
+clipboard-write
+clipboard-trim-trailing-spaces
+clipboard-paste-protection
+clipboard-paste-bracketed-safe
+title-report
+image-storage-limit
+copy-on-select
+right-click-action
+click-repeat-interval
+config-default-files
+confirm-close-surface
+quit-after-last-window-closed
+quit-after-last-window-closed-delay
+initial-window
+undo-timeout
+quick-terminal-position
+quick-terminal-size
+gtk-quick-terminal-layer
+quick-terminal-screen
+quick-terminal-animation-duration
+quick-terminal-autohide
+quick-terminal-space-behavior
+quick-terminal-keyboard-interactivity
+shell-integration
+shell-integration-features
+command-palette-entry
+osc-color-report-format
+vt-kam-allowed
+custom-shader-animation
+bell-features
+bell-audio-path
+bell-audio-volume
+app-notifications
+macos-non-native-fullscreen
+macos-window-buttons
+macos-titlebar-style
+macos-titlebar-proxy-icon
+macos-dock-drop-behavior
+macos-option-as-alt
+macos-window-shadow
+macos-hidden
+macos-auto-secure-input
+macos-secure-input-indication
+macos-applescript
+macos-icon
+macos-custom-icon
+macos-icon-frame
+macos-icon-ghost-color
+macos-icon-screen-color
+macos-shortcuts
+linux-cgroup
+linux-cgroup-hard-fail
+gtk-opengl-debug
+gtk-single-instance
+gtk-titlebar
+gtk-tabs-location
+gtk-titlebar-hide-when-maximized
+gtk-toolbar-style
+gtk-titlebar-style
+gtk-wide-tabs
+desktop-notifications
+progress-style
+faint-opacity
+async-backend
+auto-update
+auto-update-channel